Combination locks are quite common, and all mechanical ones operate in the same basic fashion. A graduated dial, typically scaled from 1 to 100, rotates a spindle that in turn rotates a drive cam. A bushing supports a series of tumblers, one of which is a drive tumbler connected to the drive cam. The drive cam drives the drive tumbler so that, when a user selects a series of combination numbers with the dial, the driven tumblers are aligned to allow operation of a door handle or lock hasp or the like. In door locks, each tumbler has a gate notch in its periphery situated such that when the notches of all of the tumblers are aligned, a stop on a door handle can travel, allowing operation of the door handle. Most combination locks have only one operative combination that is not easily changed. As a result, if the combination is compromised, a new lock with a different combination would have to be purchased, or the lock would have to be disassembled to change the combination.
In response to a desire for more flexibility and security in combination locks, changeable combination locks have been developed. Most changeable combination locks have some arrangement allowing realignment of the drive system and the tumbler gate notches to alter the combination. For example, one prior art changeable combination lock uses two-part tumblers. Inner tumblers carry teeth on their peripheries that engage teeth on the inner peripheries of outer tumblers. The outer tumblers also carry gate notches on their outer peripheries that allow use of a door handle when aligned. The inner tumblers can be disengaged from the outer tumblers to allow relative rotation for resetting the combination. To change the combination, a user throws the lock bolt out and then inserts a wire into the lock to allow relative movement of the inner and outer tumblers. The lock spindle is then pressed inward to disengage the inner tumblers from the outer tumblers, at which point the combination can be changed. This arrangement includes many steps and parts that increase the complexity of the lock. Also, the requirement of extra tools, such as a wire, to change the combination is inconvenient to the user.
Another prior art changeable combination lock includes a wave or disc spring mounted between an inner wall of a safe door and the immediately adjacent tumbler. The spring biases the tumblers against the drive cam. To change the combination of the lock, the user first removes the back panel of the lock, dials the combination to align the notches, and then holds the door handle in place to hold the notches in alignment. While holding the handle, the user pushes against the drive tumbler to disengage the drive tumbler from the drive cam. Still holding the handle and pushing against the drive tumbler, the user rotates the dial of the lock by a desired increment so that the drive cam engages the drive tumbler at a new point, thus changing the alignment point of all the tumblers by the increment the dial has been rotated. The handle and drive tumbler can then be released, and the lock can be used with the new combination. This lock is easier to use, but requires the user to do too many things at once and does not provide an easy way to disengage the drive cam and drive tumbler.
In view of the prior art, there is a need for a changeable combination lock that is simple in its construction and easy to use. The lock should allow the user to perform relatively few simple steps to change the combination of the lock.